photo: James Forsyth via flickr

DCL

Here's an interesting account on one potential effect of more glaciers melting around the world due to rising temperatures: BBC News reports that a lake under Mont Blanc glacier is about to be drained so that it doesn't flood the St Gervais valley.

The lake contains 2.3 million cubic feet of water, containing enough volume to flood the valley in 15 minutes, affecting some 900 families.

To drain the water, a team of engineers will drill through 130-165 feet of ice to reach the water cavity. Through an eight inch wide hole they will pump out the water.

Though this lake can't be directly attributed to global warming—indeed, a similar lake flooded the St Gervais valley in the late 19th century, killing 175 people—as glaciers melt, there is the very real possibility of glacial lakes forming, either under the glacier as this one or behind ice walls which could eventually break releasing torrents of water.

The latter scenario is known as a glacial lake outburst flood:

Glacial lakes come in various sizes, but may hold millions to hundreds of millions of cubic meters of water. Catastrophic failure of the containing ice or glacial sediment can release this water over a timespan of minutes to days. Peak flows as high as 15,000 cubic meters per second have been recorded in these events, suggesting that the v-shaped canyon of a normally small mountain stream could suddenly develop an extremely turbulent and fast-moving torrent some 50 metres (160 ft) deep. On a downstream floodplain, it suggests a somewhat slower inundation spreading as much as 10 kilometres (6.2 mi) wide. Both scenarios are horrific threats to lives, property and infrastructure.

Historic examples of these have occurred in Alaska, Iceland, the Contiguous United States, Canada, England, France, and throughout the Himalayas.

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